Method of Automatic Target Angle Tracking by Monopulse Radar under Conditions of Interference Distorting Location Characteristic

ABSTRACT

Method of automatic target angle tracking by sum-and-difference monopulse radar covers radiolocation sphere and specifically monopulse direction finding systems. It can be used in order to increase guidance accuracy, for example, for anti aircraft missiles and of unmanned aerial vehicles to radar targets such as: radio beacons; aerial vehicles reflecting the radio signal that illuminates them; aerial vehicles and ground-based devices radiating radio signals and jamming signals. The aim of the method consists in the assurance of reliability and stability and in the enhancement of guidance accuracy of automatic target angle tracking due to elimination of automatic tracking losses and great errors arising during the influence of the signals of orthogonal polarization or polarization close to it. 
     The proposed method provides full protection from polarization jamming for all types of monopulse radars.

The invention relates generally to radiolocation sphere, andparticularly to monopulse direction finding systems. It can be used toincrease guidance accuracy, for example, of unmanned aerial vehicles toradar targets such as: radio beacons; aerial vehicles reflecting theradio signal that illuminates them; aerial vehicles and ground-baseddevices radiating radio signals and jamming signals.

It is commonly known that the presence of antenna cross-polarizationradiation leads to reduction of direction finding accuracy; it canresult in the complete failing of the monopulse direction findingsystem, i.e. automatic tracking loss [1] (Chapters 6,8), [2]. The saidphenomenon occurs during direction finding of the targets with markeddepolarization effect which is the majority of real aerodynamic targetspossess. But this problem is most important when so-called polarizationinterference is used as electronic countermeasures means [1](paragraph8.5.2), [2], [3].

A method of target angle tracking by the sum-and-difference monopulseradio direction-finder is known, in which reception of signals from thetarget in the sum and difference channels on two orthogonal (cross)polarizations is used to decrease tracking errors (see [1], p. 249). Thedescribed direction-finders possess possibility to operate on the groupof reception channels that have polarization most closely coincidingwith the one of the reception channels.

However, the drawback of the above mentioned method is the necessity ofdoubling in the number of monopulse direction-finder reception channels(six instead of three), that makes this method virtually unacceptablefor usage in, for example, the air-borne equipment of aerial vehiclesincluding unmanned aerial vehicles (UAV) and the like due to weight andsize restrictions.

A method of target angle tracking is known, that is the closest to theclaimed one herein, which is based on the use of polarization filteringof electromagnetic waves coming from the target in thesum-and-difference monopulse radio direction-finder (see [1], p. 69-71,p. 168-169). In this case polarization filtering is performed with thehelp of the polarization array mounted in the monopulse antenna mouththat allows to weaken an adverse effect of signals on cross polarizationon the target direction finding accuracy.

However the presence of diffraction effect on the edges of thepolarization array doesn't allow to get a cross polarization level lessthan minus 35 dB (see [1], p. 165-169) with the help of polarizationfiltering which is insufficient to protect from modern polarizationinterference jammers that create interference exceeding the signal by 40dB and more (see [1], p. 224). Besides that this mode is ofteninefficient when the monopulse direction-finder antenna is located underthe blister (for example, an airplane or an unmanned aerial vehicle).The blister owing to the curvilinearity of its surface considerably (upto minus 30-minus 15 dB) increases the cross polarization level of thereceiving antenna with a polarization filter that heightens thesusceptibility of the direction-finder to the influence of polarizationinterference and leads to the degradation of target tracking accuracy(See [1], p. 158, see also [3]).

The stability analysis of the angle tracking of the polarizationinterference source by the monopulse direction-finder is published inthe transactions of the Radar Conference IEEE 2009 [4]. The trackingloss problem was brought to Lyapunov's problem about the solutionstability of a differential equation system. In this work it was shownthat the influence of polarization interference leads to negativedefiniteness of the first derivative of the direction-findingcharacteristic that results in the shift of the eigenvalue spectrummatrix of the differential equation system factors describing theautomatic control system under study in the right half-plane that in itsturn leads to the instability of the automatic tracking system and ingeneral case—to the automatic angle tracking loss. In this work it wasalso shown that it is impossible to form the optimal control functionaccording to Bellman during the operation of the angular gauge by thepolarization interference source beyond the system. Furthermore, in [4]in the state space of the automatic control system under study wascarried out the synthesis of the solution which was optimal regardingthe automatic tracking accuracy of the polarization interference jammerand it was shown the existence and uniqueness of the derived solutionwhich corresponded to the inverse function from the function of errorsignal on the condition of the detection of the polarizationinterference influence on the monopulse direction-finder.

The fact of the detection of the polarization interference influence onthe monopulse direction-finder is established by the polarizationinterference detector [5]. The polarization interference detector in thecase under consideration is an additional receiving channel of thesignals on the orthogonal polarization, the output of which with theoutput of the sum channel is supplied through detectors to thecomparator from the output of which, in the case of the detection of thepolarization interference influence, the logical unit is removed. Thisis nothing other than a polarization interference detector with asingle-bit analog-to-digital converter (See [4]).

The solution derived in [4] provides a good coincidence with thedirection-finding characteristic of the monopulse direction-finder onthe working polarization on the section approximately 0.4 . . . 0.6 ofits half-width taken as a unit (See FIG. 10) and a continuous trackingof the polarization interference source with minimum errors (See FIG. 4,line 43).

SUMMARY OF THE INVENTION

Thus, the aim and the main technical result of the present invention isto ensure stability of automatic angle tracking on target.

The set aim is achieved by the following special features:

during angle tracking by the sum-and-difference direction-finder thereception of signals from the target is performed on the fixedpolarization;

the difference signal amplitude and the phase difference between the sumand difference signals are calculated and the monopulse antenna isorientated in the direction of the target relying on the calculatedvalues of the amplitude and the phase difference signals an angularerror value and its sign;

an additional reception of signal component from the target on thepolarization, different from the working polarization of the monopulseantenna, is performed;

the amplitude values of the additional and sum signals are compared whenthe amplitude value of the additional channel signal exceeds theamplitude value of the sum signal;

the monopulse antenna is oriented relying (depending) on the angularerror, the sign of which corresponds to the measured value of the phasedifference between the sum and difference signals,

the value is formed via the inverse transformation of the measuredamplitude value of the difference signal.

The essence of the invention consists in the assurance of reliabilityand stability and in the enhancement of guidance accuracy of automatictarget angle tracking due to elimination of automatic tracking lossesand great errors arising during the influence of the signals oforthogonal polarization or polarization close to it.

The claimed method is illustrated via devices realizing thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 0 are show the flow diagram for prototype of the sum-and-differencemonopulse radio direction finder.

FIG. 1 gives the overview of the first variant of the flow diagram ofthe sum-and-difference monopulse radio direction-finder with thecomponents which realize the claimed method.

FIG. 2 shows the assumption diagrams of the directivity of the monopulseantenna, the antenna of the secondary channel and the system “monopulseantenna—antenna of the additional channel” on the working and crosspolarization. Besides, in FIG. 2 are shown time dependences of thevoltages on the outputs of the sum and secondary channels during theramp of angle α—the inclination angle of the signal polarization planein the receiving basis of the monopulse antenna with a certain constantangular velocity Ω providing the unambiguous loss of the signal source(target) automatic angle tracking by the prototype.

FIG. 3 shows time diagrams of the calculated functions of the errorsignal for the prototype and the suggested method during the ramp of theinclination angle of the signal polarization plane in the receivingbasis of the monopulse antenna with a certain constant angular velocity.

FIG. 4 shows experimental time diagrams which illustrate the radiodirection-finder principle of operation in the prototype mode and withapplication of the claimed method.

FIG. 5 shows the direction-finding characteristic on the workingpolarization at the zero inclination angle of the signal polarizationplane α.

FIG. 6 depicts the direction-finding characteristics for inclinationangles of the polarization plane α=10; 60; 70; 80; 85; 87 degrees.

FIG. 7 depicts the direction-finding characteristics for inclinationangles of the polarization plane α=88; 89; 89.5; 89.9 degrees.

FIG. 8 shows the direction-finding characteristic on the crosspolarization at α=90 degrees.

FIG. 9 depicts back unstandardized direction-finding characteristics forinclination angles of the polarization plane α=90; 89.5; 89; 88 degrees.

FIG. 10 depicts standardized back direction-finding characteristics forinclination angles of the polarization plane α=90; 89.5; 89; 88 degreesand the direction-finding characteristic on the working polarization atthe zero inclination angle of the signal polarization plane α.

FIG. 11 depicts the first variant of the diagram of the devices whichrealize the claimed method and provide an experimental check of itsproper performance.

FIG. 12 depicts the second variant of the flow diagram of thesum-and-difference monopulse radio direction-finder with the componentswhich realize the claimed method.

FIG. 13 depicts the diagram of the devices which realize operation ofthe claimed method according to the second variant.

The diagrams of the directivity of the antennas were calculated in theazimuth plane in the range of angles φε [−90; +90] degrees at a zerotilt angle (θ=0 degrees).

In the figures and in the text the following designations are used:

-   -   1—Monopulse antenna    -   2—Stripline ring    -   3—Mixer of the sum channel    -   4—Mixer of the difference channel    -   5—Heterodyne    -   6, 7—Intermediate-frequency amplifiers of the sum and difference        channels    -   8—Automatic gain control system of the sum channel    -   9—Phase detector    -   10—Error-signal amplifier    -   11—Monopulse antenna drive (mechanism)    -   12—Horn antenna of the secondary channel (waveguide aperture)    -   13—Mixer of the secondary channel    -   14—Intermediate-frequency amplifier of the secondary channel    -   15,16—Detectors of the secondary and sum channels.    -   17—Compare facility (comparator).    -   18—Switching device.    -   19—Polarization filter.    -   20—Radome.    -   21—Analog-to-digital converter.    -   22—Arithmetic unit.    -   23—Digital-to-analog converter.    -   24—F^(p) _(Σ)(φ)—assumption diagram of the mirror antenna 1        directivity of the sum channel on the working polarization in        the azimuth plane.    -   25—F^(k) _(Σ)(φ)—assumption diagram of the mirror antenna 1        directivity of the sum channel on the cross polarization in the        azimuth plane.    -   26—F^(p) _(add)(φ)—assumption diagram of the horn antenna 12        directivity of the secondary channel on the working polarization        in the azimuth plane.    -   27—F^(k) _(add)(φ)—assumption diagram of the horn antenna 12        directivity of the secondary channel on the cross polarization        in the azimuth plane.    -   28, 29—F^(p) _(Σ)(φ) and F^(k) _(add)(φ)—assumption diagrams of        the system “mirror antenna-horn antenna” at the outputs of        devices 16 and 15 respectively during operation by the target        signal on the working polarization of the mirror antenna in the        azimuth plane.    -   30, 31—F^(k) _(Σ)(φ)        F^(p) _(add)(φ)—assumption diagrams of the system “mirror        antenna-horn antenna” at the outputs of devices 16 and 15        respectively during operation by the target signal on the cross        polarization of the mirror antenna in the azimuth plane.    -   32, 33—U_(Σ)(φ,α,t)        U_(add)(φ, α,t)—calculated functions of the signals at the        outputs of devices 16 and 15 respectively during rotation of the        target signal polarization plane with a certain constant angular        velocity Ω in the basis of the receiving antenna 1 in the        azimuth plane (α=Ω=const).    -   34—U_(com)(φ,α,t)—signal at the output of comparator 17 (output        of comparator).    -   35—U_(co)(φ,α,t)—calculated function of the error signal at the        output of error-signal amplifier 10 prototype.    -   36—U_(m)(φ,α,t)—calculated function of the error signal at the        output of error-signal amplifier 10 during application of the        claimed method.    -   37—α(t)—rated dependence of the inclination angle of the target        signal polarization plane relative to the vertical line in the        receiving basis of the monopulse antenna 1.    -   38—αt)—experimental dependence of the inclination angle of the        target signal polarization plane relative to the vertical line        in the receiving basis of the monopulse antenna 1.    -   39—U_(Σ)(φ,α,t)—experimental dependence of the sum channel        voltage amplitude at the output of device 16.    -   40—U_(add)(φ,α,t)—voltage of the secondary channel at the output        of device 15.    -   41—U_(com)(φ,α,t)—voltage at the output of comparator 17.    -   42—U_(co)(φ,α,t)—experimental time dependence of the prototype        tracking error value.    -   43—U_(m)(φ,α,t)—experimental time dependence of the tracking        error value for the claimed method.

DETAILED DESCRIPTION OF THE INVENTION Example 1

The radio direction-finder (FIG. 1) comprises monopulse antenna 1 (forexample, a paraboloid of revolution with two-mode feed) in the mouth ofwhich is mounted a polarization filter 19. The working polarization forantenna 1 is a vertical one. The outputs of antenna are connected to thesum-and-difference device in the form of stripline ring 2, the sumoutput of which is connected to mixer 3 and the difference output—tomixer 4. Mixers 3 and 4 are also connected to heterodyne 5 which is alsoconnected to mixer 13. The signal input of mixer 13 is connected to hornantenna 12 having the horizontal working polarization (orthogonalrelative to the working polarization of monopulse antenna 1) and forwavelength λ aperture (mouth) area 0.5λ² . . . 1.2λ², which is mountedon the edge of antenna 1. The outputs of mixers 3 and 4 are connectedrespectively to the inputs of intermediate-frequency amplifiers 6 and 7,the outputs of which are connected to the appropriate inputs of phasedetector 9, the output of which through error-signal amplifier 10 isconnected to drive mechanism 11 of antenna 1 with polarization filter 19that is located under radome 20 and that has, for example, an ogivalform. Intermediate-frequency amplifiers 6 and 14 are connected throughdetectors 15 and 16 to the appropriate inputs of comparator (comparefacility), the output of which is connected to the driving point ofswitching device 18. The outputs of intermediate-frequency amplifiers 6and 7 are also connected to the appropriate inputs of comparator 18 theoutput of which is connected through automatic gain control system 8with intermediate-frequency amplifiers 6 and 7.

Realization of units 1-16,19 is described in the book [1] (chapters 2,3, 7).

Realization of devices 15,16,17,18 is shown in FIG. 11. Signal detectionof the secondary and sum channels in devices 15 and 16 is carried outthrough diodes D1 and D2 respectively. Comparator 17 is assembled onmicrocircuit K140UD2A (CA3047T) with bipolar feed voltageU_(feed)=±12.6+/−0.5V. Radio electronic relay 10 is used as switchingdevice 18 with operating voltage in the range [9V . . . 12V], operatingcurrent 50 mA and operating time 11 ms.

It is necessary to mention that in order to decrease operating time anytype of electronic switches on the basis of transistors, thyristors,dynistors or microcircuits instead of the relay can be used.

A device realizing the claimed method operates as follows.

Let radio direction-finder track the target the signal polarization ofwhich changes in time from the agreed polarization up to the orthogonalone in accordance with line 38 shown in FIG. 4, where α is theinclination angle of the target signal polarization vector relative tothe vertical line—the ordinate of the diagram, time is laid along theabscissa axis. The real changes of the signal polarization can be causedby the polarization interference jamming or by the fluctuations of thesignal reflected from the target. This signal after passing through theradome 20 and polarization filter 19 is received by monopulse antenna 1having the vertical working polarization. Polarization filter 19 can bein the form of a set of thin conductors located in the monopulse antenna1 mouth and oriented orthogonally to its working polarization whichprovide the reception of vertical polarization signals withoutattenuation and the reception of orthogonally polarized signals withcertain attenuation. The signals from the outputs of monopulse antenna 1come to the inputs of stripline ring 2 providing at its outputs theshaping of microwave signals of the sum and difference channels thesignals of which come to mixers 3 and 4 respectively where they aretransformed with the help of heterodyne 5 into the signals ofintermediate frequency, which then are amplified inintermediate-frequency amplifiers 6 and 7 up to the required value andcome to the inputs of phase detector 9. The difference signal amplitudedetermines the value of the angular error signal at the output of phasedetector 9, the phase difference at the input of phase detector 9between the signals of the sum and difference channels determines thesign of the angular error signal U_(co)(φ,α,t) at the output 9 where φis the angular error (displacement angle between a true direction ontarget and radar boresight of the monopulse direction-finder), α is theinclination angle of the target signal polarization vector relative tothe working polarization vector of the monopulse antenna, and t is atime. Automatic gain control system 8 excludes the dependence of theangular error signal amplitude at the output of phase detector 9 on thelevel of the received signals by the connection of the input ofautomatic gain control system 8 through normally closed contacts ofswitching device 18 to the output of intermediate-frequency amplifier 6of the sum channel, in this case the signal at the output of automaticgain control system 8 makes a simultaneous adjustment of theamplification coefficients of intermediate-frequency amplifiers 6 and 7providing the signal normalization of the difference channel with thehelp of the sum one.

At the same time the reception of the signal component on the horizontalpolarization by the secondary channel of the direction-finder isperformed with the help of horn antenna 12, mixer 13 andintermediate-frequency amplifier 14.

Time dependences of the voltages on the outputs of the sum channel U_(Σ)(φ,α,t) and secondary channel U_(add)(φ,α,t) are shown in FIG. 2 withcurves 32 and 33 respectively. Voltage of automatic gain control systemin dB (sum channel) is shown in FIG. 4 by curve 39 and the signal of thesecondary channel—by line 40.

Voltage U_(com)(φ,α,t) at the output of comparator 17 (FIG. 4 line 41)will be equal to +U_(feed), when U_(add)(φ,α,t)>U_(Σ) (φ,α,t) and willbe equal to −U_(feed) when U_(add)(φ,α,t)<U_(Σ). (φ,α,t):

${U_{com}\left( {\phi,\alpha,t} \right)} = \left\{ \begin{matrix}{{+ U_{feed}},} & {{{when}\mspace{14mu} {U_{add}\left( {\phi,\alpha,t} \right)}} > {U_{\Sigma}\left( {\phi,\alpha,t} \right)}} \\{- U_{{feed},}} & {{{when}\mspace{14mu} {U_{add}\left( {\phi,\alpha,t} \right)}} < {U_{\Sigma}\left( {\phi,\alpha,t} \right)}}\end{matrix} \right.$

If the leg 1 of microcircuit K140UD2A is grounded the necessity in diodeD3 disappears. The voltage at the output of comparison (comparator)circuit 17 is shown in FIG. 2 by line 34 and is written in the followingform:

${U_{com}\left( {\phi,\alpha,t} \right)} = \left\{ \begin{matrix}{{+ U_{feed}},} & {{{when}\mspace{14mu} {U_{add}\left( {\phi,\alpha,t} \right)}} > {U_{\Sigma}\left( {\phi,\alpha,t} \right)}} \\{0,} & {{{when}\mspace{14mu} {U_{add}\left( {\phi,\alpha,t} \right)}} < {U_{\Sigma}\left( {\phi,\alpha,t} \right)}}\end{matrix} \right.$

Voltage of the automatic gain control system, curve 32, and voltage ofthe secondary channel, curve 33, is shown in dB in FIG. 4, andU_(com)(φ,α,t)—in volts. Time is shown on the abscissa axis.

Voltage U_(com)(φ,α,t) comes to switching device 18 as a control signal.

FIG. 4 shows the operation of the radio direction-finder in theprototype mode and in the mode of the claimed method.

Operation of Prototype Mode

-   Conditions: —power supply to device 17 is switched off (microcircuit    K140UD2A is disconnected);    -   relay R1 contacts are normally closed.        Operation order is shown in FIG. 4:

Up to time point t₁, the following condition is fulfilled:

U _(Σ)(φ,α,t)>U _(add)(φ,α,t)

a control signal at the input of switching device 18 is absent (line 41in FIG. 4) and the direction-finder works in the prototype mode—in thedesign mode of automatic target tracking [1](p. 69-71). The input ofautomatic gain control system 8 is connected through normally closedcontacts of relay R1 (switching device 18) to the output ofintermediate-frequency amplifier 6 of the sum channel whereby the signalnormalization of the difference channel is carried out with the help ofthe sum one. The error signal from the output of phase detector 9through error-signal amplifier 10 comes to drive mechanism 11 of themonopulse antenna which turns the antenna in such a way that its radarboresight coincide with the direction on target and the error signalvalue is maintained close to zero. As the inclination angle of thetarget signal polarization plane of the input signal reaches theorthogonal position the voltage amplitude of automatic gain controlsystem 8 decreases (FIG. 4, curve 39) and after a certain value startsthe avalanche-like increase of the error signal (FIG. 4, curve 42).

In time interval t₁<t<t₂ the target signal polarization vector passesthrough the position close to the orthogonal position which is relativeto the working polarization of antenna 1 (see FIG. 4, curve 38). In thiscase at the output of phase detector 9 abruptly increases the angletracking error which leads to the loss of automatic angle tracking ontarget. The sum and difference channels change places, normalizationcondition is violated (See [1] Sections 7.3, 8.5). The automatictracking loss occurs because during the impact of the signal on theorthogonal polarization on the monopulse direction-finder the voltage ofthe sum channel reaches in a certain small ε-neighborhood of the radarboresight the values close to zero and, being in the denominator, turnsthe error signal into infinity.

The Claimed Method Operation.

-   Conditions: —power supply to device 17 is switched on (microcircuit    K140UD2A is switched on);    -   contacts of switching device (relay R1) are normally closed.        Operation procedure is shown in FIG. 4:        During application of the claimed method the monopulse        direction-finder operates in the prototype mode (in the design        mode) up to time point t₆:

the following condition is met: U_(Σ)(φ,α,t)>U_(add)(φ,α,t);

at the output of device 17 the control voltage is absentU_(com)(φ,α,t)=0.

the input of automatic gain control system 8 is connected throughnormally closed contacts of relay R1 (switching device 18) to the outputof intermediate-frequency amplifier 6 of the sum channel whereby thesignal normalization of the difference channel is carried out with thehelp of the sum one.

At interval t₆<t<t₇:

U_(Σ) (φ,α,t)<U_(add)(φ,α,t);

at the output of device 17 the control voltage is generatedU_(com)(φ,α,t)

under the influence of the control voltage from comparator 17U_(com)(φ,α,t) switching device 18 is actuated: it disconnects the inputof automatic gain control system from the output ofintermediate-frequency amplifier 6 of the sum channel and connects theinput of automatic gain control system 8 to the output ofintermediate-frequency amplifier 6 whereby the signal normalization ofthe sum channel is carried out with the help of the difference channeland the decision derived in [4] is realized. In time interval t₆<t<t₇the loss of automatic angle tracking on target doesn't occur because atthe time of the signal influence on cross polarization in time intervalt₆<t<t₇ due to application of devices 12-18 drive mechanism 11 carriesout orientation of antenna 1 on target according to thedirection-finding characteristic close to the direction-findingcharacteristic on the working polarization. In this case the voltage ofthe difference channel which can reach in a certain smallε-neighbourhood of the radar boresight sufficiently big values appearsin the denominator, and the values of the sum channel close to zeromoves to the numerator.

When the polarization plane passes the signal of the orthogonal positionthe voltage of the difference channel decreases due to the change of thedirectivity diagram, the amplification coefficient increasescorrespondingly (desensitization decreases) of the sum and differencechannels respectively. During this process the amplitudes of the sum andsecondary channels are permanently compared. After passing point t₇:

the following condition is met: U_(Σ) (φ,α,t)>U_(add)(φ,α,t);

at the output of device 17 the control voltage is absentU_(com)(φ,α,t)=0

switching device 18 is actuated: it disconnects the input of automaticgain control system from the output of intermediate-frequency amplifier7 of the difference channel and returns the connection of the input ofautomatic gain control system 8 to the output of intermediate-frequencyamplifier 6 of the sum channel whereby the standard normalization of thedifference channel signal is carried out with the help of the sumchannel.

The circuit consisting of devices 12-17 can be characterized as asingle-bit detector of the interference on the cross polarization, anddevice 18 connecting by the signal of the interference polarizationdetector the input of automatic gain control system 8 to the output ofintermediate-frequency amplifier 6 of the sum channel or to the outputof intermediate-frequency amplifier 7 of the difference channel as aprotector of the monopulse direction-finder from the impact ofcross-polarization signals and interferences.

Example 2

The radio direction-finder (FIG. 12) includes monopulse antenna (forexample, a paraboloid of revolution with two-mode feed) in the mouth ofwhich polarization filter 19 is mounted. The working polarization forantenna 1 is a vertical one. The outputs of antenna are connected to thesum-and-difference device in the form of stripline ring 2, the sumoutput of which is connected to mixer 3 and the difference output—tomixer 4. Mixers 3 and 4 are also connected to heterodyne 5 which is alsoconnected to mixer 13. The signal input of mixer 13 is connected to hornantenna 12 having the horizontal working polarization (orthogonalrelative to the working polarization of monopulse antenna 1) andaperture (mouth) area 0.5λ² . . . 1.2λ², which is mounted on the edge ofantenna 1. The outputs of mixers 3 and 4 are connected respectively tothe inputs of intermediate-frequency amplifiers 6 and 7. The output ofintermediate-frequency amplifier 6 is connected to the input ofautomatic gain control system 8 the output of which is connected tointermediate-frequency amplifiers 6 and 7. The outputs ofintermediate-frequency amplifiers 6 and 7 are connected to phasedetector 9, and the outputs of intermediate-frequency amplifiers 6 and14 are connected through detectors 15 and 16 to the corresponding inputsof comparator 17 the output of which is connected to the control inputof switching device 18. The output of phase detector 9 is connected tothe signal input of switching device 18, one output of which isconnected to drive mechanism 11 of antenna 1 through error-signalamplifier 10, the other output of the switching device throughanalog-to-digital converter 21, arithmetic unit 22, digital-to-analogconverter 23 and error-signal amplifier is also connected to drivemechanism 11 of antenna 1 located under radome 20 and having, forexample, an ogival form.

Realization of units 1-16,19 is described in [1] chapters 2, 3, 7.

Realization of devices 15,16,17,18, 19, 20, 21 is shown in FIG. 13.Devices 15,16,17 and 18 are described above. As device 21 aneight-digits analog-to-digital converter on microcircuit K1107PV4A (TDC1025J) with the range of input voltage [−2.5V . . . +2.5V] was used,programmable read-only memory KR556RT5 was used as arithmetic unit 22,as eight-digits digital-to-analog converter (device 23)-microcircuit1118 PA1 (MS 10318).

A device realizing the claimed method operates in accordance with thefollowing method.

Let radio direction-finder track the target, the signal polarization ofwhich changes in time from the agreed polarization up to the orthogonalone in accordance with line 37 shown in FIG. 3, where a is theinclination angle of the target signal polarization vector relative tothe vertical line—the ordinate of the diagram, time is laid along theabscissa axis. The real changes of the signal polarization can be causedby the polarization interference jamming or by the fluctuations of thesignal reflected from the target. This signal after passing throughradome 20 and polarization filter 19 is received by monopulse antenna 1having the vertical working polarization. The polarization filter can bein the form of a set of thin conductors located in the monopulse antenna1 mouth and oriented orthogonally to its working polarization whichprovide the reception of vertical polarization signals withoutattenuation and the reception of orthogonally polarized signals withcertain attenuation. The signals from the outputs of monopulse antenna 1come to the inputs of stripline ring 2 providing at its outputs theshaping of microwave signals of the sum and difference channels thesignals of which come to mixers 3 and 4 respectively where they aretransformed with the help of heterodyne 5 into the signals ofintermediate frequency, which then are amplified inintermediate-frequency amplifiers 6 and 7 up to the required value andcome to the inputs of phase detector 9. The difference signal amplitudedetermines the value of the angular error signal at the output of phasedetector 9, the phase difference at the input of phase detector 9between the signals of the sum and difference channels determines thesign of the angular error signal at the output of phase detector 9.Automatic gain control system 8 excludes the dependence of the angularerror signal amplitude at the output of phase detector 9 on the level ofthe received signals by the connection of the input of automatic gaincontrol system 8 to the output of intermediate-frequency amplifier 6 ofthe sum channel, in this case the signal at the output of automatic gaincontrol system 8 makes a simultaneous adjustment of the amplificationcoefficients of intermediate-frequency amplifiers 6 and 7 providing thesignal normalization of the difference channel with the help of the sumone.

Simultaneously is carried out the reception of the signal component onthe horizontal polarization by the secondary channel of thedirection-finder with the help of horn antenna 12, mixer 13 andintermediate-frequency amplifier 14.

Time dependences shown in FIGS. 2, 3 and 4 are the same. Expressions arealso true for U_(com)—the voltage at the output of comparator 17.

a) Prior Art Operation (Prototype Mode)

-   Conditions: —power supply to device 17 is switched off (microcircuit    K140UD2A show in FIG. 13 is switched off);    -   off); contacts of switching device 18 (relay R1 show in FIG. 13)        are normally closed.        Operation procedure is shown in FIG. 4:

Up to time point t₁, the following condition is fulfilled:

U _(Σ)(φ,α,t)>U _(add)(φ,α,t)

a control signal at the input of switching device 18 is absent (line 41in FIG. 4) and the direction-finder works in the prototype mode—in thedesign mode of automatic target tracking ([1] p.p. 69-71). The errorsignal from the output of phase detector 9 through the normally closedcontacts of switching device 18 comes to error-signal amplifier 10 andthen to drive mechanism 11 of the monopulse antenna which turns antenna1 in such a way that its radar boresight coincides with the direction ontarget and the error signal value is maintained close to zero. As theinclination angle of the target signal polarization plane of the inputsignal reaches the orthogonal position the voltage amplitude ofautomatic gain control system decreases and after a certain value startsthe avalanche-like increase of the error signal.

In time interval t₁<t<t₂ the target signal polarization vector passesthrough the position close to the orthogonal position which is relativeto the working polarization of antenna 1 (see FIG. 3, curve 41). In thiscase at the output of phase detector 9 abruptly increases the angletracking error which leads to the loss of automatic angle tracking ontarget. (See [1], Sections 7.3, 8.5).

b) Claimed Method Operation

-   Conditions: —power supply to device 17 is switched on (microcircuit    L140UD2A is switched on);    -   contacts of switching device 18 (relay R1 shown in FIG. 13) are        normally closed.        Operation procedure is shown in FIG. 4:

When the claimed method is used the loss of automatic angle tracking ontarget doesn't occur because at the time of the signal influence oncross polarization in time interval t₆<t<t₇ due to application ofdevices 12-23 drive mechanism 11 carries out orientation of antenna 1 ontarget according to the direction-finding characteristic close to thedirection-finding characteristic on the working polarization (See FIG.10). It is achieved by the use of the control function U_(contr)(t)calculated with the help of arithmetic unit 22 realized on theprogrammable read-only memory which carries out a table functionaltransformation of the error signal function U_(co)(φ,α,t) having thefollowing form:

U _(m)(φ,α,t)=U _(contr)(t)=[U _(co)(φ,α,t)]⁻¹

As it is seen from FIG. 4 (curve 43) the angular error valueU_(m)(φ,α,t) in time interval t₆<t<t₇ doesn't exceed the value. At timepoint t₇, when the target signal polarization vector finishes to passthrough a hazardous position (FIG. 4, curve 38), the control voltage atthe input of switching device 18 turns into zero (curve 41) andswitching device 18 disconnects phase detector 9 from the circuit ofdevices 19-21 and connects it directly to error-signal amplifier 10 andto drive mechanism 11 of antenna 1, the direction-finder returns tooperation in the design mode of automatic tracking in which the errorsignal from the output of phase detector 9 is used to operate antenna 1tracking the target.

The circuit consisting of devices 12-17 can be characterized as asingle-bit detector of the interference on the cross polarization, andthe circuit of devices 18, 21-23 as a protector of the monopulsedirection-finder from the impact of cross-polarization signals andinterferences.

Application of the invention will allow to:

Reduce the direction-finding error caused by the depolarization of thesignals reflected from the target to a minimum;

Exclude losses of automatic angle tracking on target of the polarizationinterference jammer;

Increase target tracking accuracy of the polarization interferencejammer in 8-10 times.

It should be mentioned that a positive effect is greater when thedirection-finder antenna is mounted under the blister.

A additional significant advantage of the method is the fact that itshardware implementation is based on cheap parabolic antennas and itdoesn't require a great volume of additional equipment. When the claimedmethod is used it is unnecessary to mount on an aerial vehicle(including an unmanned aerial vehicle) expensive flat antenna arrays asmonopulse antenna 1 which are used as the solution of the hazards ofautomatic angle tracking loss caused by the influence of the signals oncross polarization.

Some additional useful remarks and applications of the disclosed methodand devices are described in details in [6].

CITED DOCUMENTS

[1]. A. I. Leonov, K. I. Fomichev. Monopulse radiolocation. Moscow,Radio and communication, 1984.

[2]. A. I. Leonov, K. I. Fomichev. Monopulse Radar. Artech House. 1986.

[3]. Van Brunt L. B. Applied ECM N.Y., 1978, v. 1, E.W.Enginering. Part4.

[4]. E. Markin, On interference immunity of angle tracking systems underconditions of interference distorting location characteristic, RadarConference, 2009 IEEE 4-8 May 2009 Pages: 1-6, Pasadena, USA, DigitalObject Identifier 10.1109/RADAR.2009.4977092.

[5]. E. Markin, Jamming detection in providing for radar jammingimmunity, EUROCON'09 IEEE, May 18-23, 2009, Pages: 1565-1567, SaintPetersburg, Russia, Digital Object Identifier10.1109/EUROCON.2009.5167849.

[6]. E. Markin, Method of automatic target angle tracking bysum-and-difference monopulse radar invariant against the polarizationjamming. Intellcom LLC, Moscow, Russian Federation. EUROPWEAN MICROWAVEWEEK 2010, CNIT La Defense, Paris, France, September 26—01 Oct. 1 2010.Conference Program, page 75: Sep. 30 2010, EuRAD Poster05-6.

1. A method of automatic target angle tracking by the sum-and-differencemonopulse radio direction-finder, said method comprising at least thefollowing steps: the receiving signals from the target by the monopulseantenna on the fixed polarization angle; the difference signal amplitudeis measured; the phase difference value between the sum signal anddifference signal is calculated; the monopulse antenna is orientated inthe direction of the target, said direction is calculated using saidcalculated values of the amplitude as the angular error value and thephase difference sign as and the angular error sign; receivingadditional signal component from the target on the differentpolarization direction, said different polarization direction beingdifferent in direction from that of working polarization of saidmonopulse antenna; a difference value is calculated by subtractingamplitude value of said signal component from the amplitude value of thesum channel; during the time interval when said difference value is lessthan a zero the orientation of the monopulse antenna is performedrelying on the value of angular error, the sign of said angular errorcorresponds to the said phase difference value between said sum and saiddifference signals, and the value of said angular error is the formed byreverse conversion of the said signal amplitude values.
 2. A radiodirection-finder comprises monopulse antenna, preferably a paraboloid ofrevolution with two-mode feed, with the vertical working polarization;polarization filter mounted in the mouth of said monopulse antenna; theoutputs of said monopulse antenna are connected to thesum-and-difference device in the form of stripline ring or hybridT-joint; the sum output of said sum-and-difference device is connectedto the first mixer and the difference output is connected to the secondmixer; first and second mixers are also connected to heterodyne; saidheterodyne is also connected to third mixer; the signal input of saidthird mixer is connected to horn antenna having the horizontal workingpolarization, orthogonal relatively to the working polarization of saidmonopulse antenna and aperture (mouth) area 0.5 . . . 1.2λ²; said hornantenna is mounted on any convenient place of said monopulse antenna;the outputs of said first and second mixers are connected to the inputsof the first and second intermediate-frequency amplifiers respectively;the outputs of said first and second intermediate-frequency amplifiersare connected to the appropriate inputs of phase detector; the output ofsaid phase detector through error-signal amplifier is connected to drivemechanism of said monopulse antenna with polarization filter; saidpolarization filter is located under radome; first and thirdintermediate-frequency amplifiers are connected through first and seconddetectors to the appropriate inputs of a compare means (comparator); theoutput of said g device; the outputs of said first and secondintermediate-frequency amplifiers are also connected to the appropriateinputs of said compare means the output of said automatic gain controlsystem with said first and second intermediate-frequency amplifiers. 3.The A radio direction-finder as recited in claim 2, where saidpolarization filter has an ogival form.
 4. The device as recited inclaim 2, where said horn antenna is mounted on the edge of saidmonopulse antenna
 5. The radio direction-finder comprises monopulseantenna, preferably a paraboloid of revolution with two-mode feed, withpolarization filter mounted in the mouth thereof, said monopulse antennaworking polarization is vertical; sum-and-difference device in the formof stripline ring or hybrid T-joint connected to outputs of saidmonopulse antenna; the sum output of said sum-and-difference device(means) is connected to first mixer and the difference output of saidsum-and-difference device (means) is connected to second mixer; saidfirst and second mixers are connected to heterodyne which is connectedto third mixer; the signal input of third mixer is connected to hornantenna having the horizontal working polarization orthogonal relativelyto the working polarization of said monopulse antenna and aperture(mouth) area 0.5 . . . 1.2λ²; the outputs of said first and secondmixers are connected to the inputs of first and secondintermediate-frequency amplifiers, respectively; the output of the firstintermediate-frequency amplifier is connected to the input of automaticgain control system; the output of said automatic gain control system isconnected to said first and second intermediate-frequency amplifiers;the outputs of said first and second intermediate-frequency amplifiersare connected to a phase detector, and the outputs of first and thirdintermediate-frequency amplifiers are connected through first and seconddetectors to the corresponding inputs of comparator; the output of saidcomparator is connected to the control input of switching device; theoutput of phase detector is connected to the signal input of saidswitching device; the first output of said switching device is connectedto drive mechanism of said monopulse antenna through error-signalamplifier; the second output of said switching device throughanalog-to-digital converter, arithmetic unit, digital-to-analogconverter and said error-signal amplifier is connected to said drivemechanism of said monopulse antenna.
 6. The device as recited in claim5, where said monopulse antenna is located under radome.
 7. The deviceas recited in claim 5, where said monopulse antenna and having an ogivalform.
 8. The device as recited in claim 5, where said horn antenna ismounted on the any convenient place of monopulse antenna.
 9. The deviceas recited in claim 5, where said horn antenna is mounted on the edge ofsaid monopulse antenna.